Apparatus for electrolytic surface treatment

ABSTRACT

A METHOD AND AN APPARTUS FOR CARRING OUT THE METHOD OF ELECTROCHEMICAL SURFACE TREATEMENT OF METALLIC OBJECTS, THE METHOD BEING WITH BIPOLAR CURRENT CONDUCTION IN SEPARATE ELECTROYSIS CELLS COMBINED WITH THE METHOD EMPLOYING AN ELECTRODE GAP DEREASING IN THE DIRECTION OF MOVEMENT OF THE OBJECT FOR THE PURPOSE OF ACHIEVEING MAXIMUM ELECTROCHEMICAL EFFICIENCY, MAXIMUM ECONOMY, AND OPERATION RELIABILITY.

2 Sheets-Sheet 1 INVENTOI? Bengt Ivor Andersson ATTORNEYS.

March 1972 B. ANDERSSON APPARATUS FOR ELECTROLYTIC SURFACE TREATMENT Filed May 5, 1969 March 21, 1972 ANDERSSON 3,650,935

APPARATUS FOR ELECTROLYTIC SURFACE TREATMENT Filed May 5, 1969 2 Sheets-Sheet 2 INVENTOR Bengt Ivor Andersson ATTORNEYS.

United States Patent C) 3,650,935 APPARATUS FOR ELECTROLYTIC SURFACE TREATMENT Ben t Ivar Andersson, Torshalla, Sweden, assignor to AB C. J. Wennbergs Mekaniska Verkstad, Karlstad, Sweden Filed May 5, 1969, Ser. No. 821,691 Claims priority, application Sweden, May 6, 1968, 6,047/ 68 Int. Cl. C23b 1/04, 5/58; B01k 3/00 US. Cl. 204206 Claims ABSTRACT OF THE DISCLOSURE A method and an apparatus for carrying out the method of electrochemical surface treatment of metallic objects, the method being with bipolar current conduction in separate electrolysis cells combined with the method employing an electrode gap decreasing in the direction of movement of the object for the purpose of achieving maximum electrochemical efiiciency, maximum economy, and operation reliability.

In industrial electrochemical processes which are customary at the present time, for example the pickling of metallic material in sheet or strip form, acceptable results are obtained provided that the widths are not great. As soon as greater widths of sheet or strip have to be pickled, problems arise which so far it has not been possible to solve satisfactorily and particularly in an economical manner.

The reason is that conventional concepts of electrochemical surface treatment acquired with narrow strips were simply applied to greater strip widths and at the same time equipment was adapted to these widths. Starting from a determined width these extrapolated processes are no longer practicable.

To mention some of the factors which prevent the application of these extrapolated methods, the increasing space required for plant with increasing width of strip is a primary consideration. Secondly, the amounts of electrolyte required become so great that, among other things, technical manufacturing difliculties occur in the handling of these quantities, that is to say regeneration becomes practically impossible if the process is to be carried out economically.

The reaction products which are produced in the electrolyte during the pickling process, in the form of sludge or colloidal particles, have a disturbing effect on the pickling and must therefore be removed if acceptable pickling results are to be obtained. This applies very particularly to a process which in itself is technically valuable, namely the continuous pickling of stainless steel in strip form, for which purpose neutral salt solutions are used as electrolyte. In the case of this method metal ions occurring in the form of hydroxides are dissolved in the course of the process out of the mill scale layer which is to be removed, while these metal hydroxides either remain suspended in the electrolyte or form the abovementioned sludge. This sludge and the suspended metal hydroxide particles disturb the pickling process. However, they also tend to become secured to the surface of the strip if the latter passes with anodic polarity out of the last electrolysis cell. This anodic polarity of the strip is necessary in order to achieve complete passivity of the surface of the stainless steel. In order to remove these reaction products adhering to the steel surface, it is neces- 3,650,935 Patented Mar. 21, 1972 sary in this process to provide the plant with expensive brushing equipment and to eifect chemical afterpickling after the neutral pickling.

In order to remove the reaction products from the electrolyte it is necessary to replace the latter continuously or to regenerate it. If it is desired to achieve eflFective continuous regeneration of the electrolyte during the treatment, the eletcrolyte must be given a volume such that complete cleaning or regeneration is possible.

In the pickling of stainless strip of great width, for example, it would therefore be necessary to use electrolyte volumes which in the present state of the art would make it impossible to carry out effective continuous cleaning of the electrolyte in an economical manner. At the present time therefore plants are used the volumes of which are below this limit dictated by economy. Even in these plants however relatively large amounts of electrolyte are used, whereby the concentration of the reaction product is reduced and an economical operating time achieved. Another method consists in periodically pickling and regenerating. It is also possible to use two lines, one line alternately pickling and the other regenerating. By these solutions however it has not been found possible to solve the problem which arises as soon as very great widths of strip have to be pickled. The electrolyte tanks, pickling vessels, and regeneration systems, etc. required for this purpose according to the conventional concept are no longer economically acceptable.

In addition to the abovementioned difiiculties which make it difficult or even impossible to employ these electrochemical pickling processes, there are other important factors which must be taken into account. One of these is the occurrence of reaction gases, which have a decisive influence on the performance of the electrochemical process and the result of the surface treatment. These gases must be rapidly and effectively removed from the reaction chamber, that is to say from the space between the electrodes and the strip. Vigorous movement of the bath during the treatment is necessary for this purpose. In the pickling of narrow strip it was possible, as stated above, to solve this problem and to comply satisfactorily with the requirement of freedom from gas. In the case of strip widths in the limit region, for which very large amounts of electrolyte must be used, this problem however can be solved both from the technical and from the economical point of view only with very great difficulty.

.Finally, mention should be made of the arrangement of the electrodes, which according to the conventional concept are spaced relatively great distances apart in order thereby to facilitate the removal of the abovementioned gases. At the same time, it is desired thereby to ensure a suflicient amount of relatively new electrolyte at the treatment point.

The aim of the present invention is to eliminate the abovementioned disadvantages and limitations in the electrolytic pickling of strip, particularly wide strip. According to the invention, this is achieved by a modification of the method hitherto applied and by a special construction of the equipment for carrying out this method.

The fundamental novelty of this method is the use of a directed electrolyte flow, which continuously supplies to the treatment point the minimum amount of fresh electrolyte necessary for each width of strip. In addition, the prerequisites for effective removal of the gases formed in the course of the process are complied with. The reaction chamber is relatively small, which is achieved through the fact that the electrodes are disposed short distances apart and form narrow gaps, thus resulting in an automatically reduced volume of electrolyte and consequently in treatment equipment which is automatically simplified to an extremely great extent, particularly for the regeneration. The method according to the invention is therefore characterised in that the method of bipolar current conduction in separate electrolysis cells is combined with the method employing an electrode gap which decreases in the direction of movement of the object, for the purpose of achieving maximum electrochemical elficiency, maximum economy, and operational reliability.

The apparatus for carrying out this method is characterised in that the electrolysis cells are in the form of wedge-shaped boxes in which the roof and the bottom, which are straight or curved in shape, are inclined in the direction of movement of the object and are provided with electrodes fastened on them or with electrodes which form the roof and the floor, while each cell forms a closed circuit for the movement of the electrolyte, with the inlet for the gas-free, clean electrolyte on the side where the gap between the electrodes is the narrowest, and with the outlet for the used electrolyte on the side where the gap between the electrodes is the widest, the object to be treated being passed through these cells oppositely to the direction of flow of the electrolyte.

This apparatus corresponding to the method of the invention thus entails above all a considerable reduction of the amount of electrolyte and thus also of the dimensions of the plant, in respect of width, height, and also length. This results in a considerable saving in cost of plant and operation. The narrow gap between the electrodes and the object to be treated, in combination with the directed electrolyte fiow, results in greater electrochemical effect, that is to say the current densities required in conventional methods can be obtained through the narrow gap with a lower power input, or in other words with the amounts of energy which are used at the present time in conventional methods it is possible according to the invention to obtain a current density increased a number of times. The directed electrolyte flow leads to complete removal of the larger amounts of gas formed at the higher current densities and eliminates the extremely harmful effect of these quantities of gas on the pickling process. The reduced amount of electrolyte according to the invention can be treated with ease, that is to say can be cleaned or regenerated by conventional methods.

The invention is explained more fully below with the aid of technical data (see Table 1) and examples (see FIGS. 1 to 3).

The amount of electrolyte, which is considerably increased by the method and apparatus of the invention, together with the likewise considerably increased efiectiveness of the pickling reaction can be seen clearly and without ambiguity from the comparative values given in Table 1 for quantity proportions in the pickling of strip up to a width of 1400 mm. The width was restricted to 1400 mm. because of the amount of electrolyte which is used in this case if the conventional method is employed, namely 1800 litres of electrolyte. If it is desired to maintain continuous operation with 1800 litres of electrolyte for each cell, a difficult problem will be encountered in respect of regeneration and movement of the bath. At the present time there are standard widths of strip up to 2000 mm. For the pickling of these widths of strip it would be necessary to prepare 3000 litres of electrolyte for each cell if the conventional method were employed. Because of the considerable space required for installing the necessary equipment for this regeneration, continuous cleaning of 3000 litres of electrolyte is very inconvenient. According to the invention, only 360 litres of electrolyte are required for this width of strip, that is to say one-tenth of the amount otherwise necessary for the conventional method.

It can for example be seen from the table that in the pickling of strip of a width of 1400 mm. (cell length 1000 mm.) by the conventional method, about 900 litres of electrolyte are required. When the method according to the invention is employed the amount of electrolyte required is reduced to one-tenth, namely 97.5 litres. This is a consequence of the construction of the electrolysis cell, which in the case first mentioned has the following dimensions: length 1000 mm., width 1500 mm., height 600 mm. The electrolysis cell according to the invention has a length of 1000 mm., a width of 1500 mm., and two different gap widths, that is to say heights, namely mm. at the outlet and 30 mm. at the inlet. The saving of electrolyte is therefore 800 litres. It can clearly be seen from the table that the method and apparatus according to the invention eliminate the limitations existing at the present time in respect of electrochemical pickling.

When the method according to the invention is employed, it is possible to pickle practically any width of strip, because the electrolyte flowing through the narrow gap oppositely to the direction of movement of the object has equal freshness over the entire width of the strip and uniform performance of the pickling is ensured at every point of the surface of the strip.

In the accompanying drawings:

FIG. 1 is a side elevation of a pickling line according to the invention;

FIG. 2 is a plan view of the same plant, and

FIG. 3 is a section through an electrolysis cell according to the invention.

The plant illustrated in the drawings consists of a number of electrolysis cells 10, which are of the special form of construction illustrated in FIG. 3 and have bipolar current conduction, that is to say each cell is either the anode or the cathode in alternating sequence. The strip 1, which comes from a reel (not shown), passes through the cells to a take-up reel and is supported by supporting and transport rollers 9. In the chamber 8 the waste electrolyte accumulates for example from the seals 3.

The electrodes, which are alternatively cathode 2 and anodes 6, form the roof and floor of the cells and are inclined in the direction 13 of the movement of the strip. The distances a between the strip and the electrodes !become increasingly smaller in the direction of the movement of the strip and attain the smallest gap width at the electrolyte inlet 7. Clean, gas-free electrolyte flows through the inlet 7 in the direction 12 oppositely to the movement of the strip and first passes through the narrowest gap aperture. The used electrolyte accumulates in the vessel 4 and is passed through the outlet 5 to a plant 14 for regeneration, heating or cooling, and gas removal.

Brushing devices, such as are used for example in conventional methods, are not required, this being another advantage of the method and apparatus according to the invention, resulting in turn in a considerable reduction of the cost of the plant, the space required, and operating costs.

To sum up, it may be stated that by the method according to the invention great advantages are achieved in process technique, great savings of the cost of plant, construction, and operation, and great operational reliability are achieved. The higher quality of the pickled products, and also the economy of the process are achieved without measures entailing expense.

A consequence of the high electrochemical efiiciency which, among other things, is achieved by means of the method of the invention, consists in that the method of the invention can likewise be used for electrochemical metal deposition.

An aqueous solution consisting of -200 grams per litre of sodium sulphate can be used as electrolyte for pickling.

An acid copperplating bath containing 210 grams per litre of copper sulphate, 37.5 grams per litre of sulphuric acid, and optionally an addition comprising a wetting agent and/or glossing agent is suitable as electrolyte for the deposition of metal, for example copper.

having electrolyte inlet means and outlet means, said electrode means forming current concentration means for establishing the greatest current density through the electrolyte at the portion of the electrode means closest TABLE L-QUANTITY PROPORTIONS IN THE PICKLING OF STRIP WIDTHS UP TO 1,400 MM.

Electrolysis cells according to invention Difierence Conventional electrolysis cells Height in quan- Quantity tities of Length, Width, Height, Amount, a1, a2, Amount, proportions, electrolyte, mm. mm. mm. litres Length Width mm. mm. litres U/K litres NOTE.-K= Conventional electrolysis cells; U=Electrolysis cells according to the invention; the indication height of conven tional electrolysis cells" relates to the level of the electrolyte (the height of the cell is generally 1 metre); cell height according to the inventioua1=greatest gap width (outlet gap for the electrolyte); a2=smallest gap width (inlet gap for the electrolyte).

What is claimed is:

1. An apparatus for the electrochemical treatment of a metal strip, comprising a plurality of electrolysis cells in the form of wedge-shaped boxes with parallel side walls, means for passing the strip through the electrolysis cells from the wide to the narrow end of the cells, electrodes at the top and bottom walls of the cells, the electrodes of each cell having electrode ends and defining a wedgeshaped chamber between the electrodes ends within the cell, means for supplying electric current to the electrodes, the electrodes of each alternate cell being coupled as anodes relative to the strip, the electrodes of each cell between the alternate cells being coupled as cathodes relative to the strip, electrolyte inlet means at the narrow end of the cells, electrolyte outlet means at the broad end of the cells, a regenerating plant for the electrolyte, means for pumping the used electrolyte from the wide end of the cells to the regenerating plant, and means for pumping the regenerated electrolyte from the regenerating plant to the narrow end of the cells.

2. An apparatus as in claim 1, wherein the electrodes of the last cell, in the direction of movement of the strip, are connected as cathodes to the current source.

3. An apparatus for electrochemical treatment of a metal strip, comprising a plurality of electrolysis cells, drive means for passing the strip through the cells in a first direction, pump means for passing an electrolyte through each cell in a second direction opposite the first direction, electrode means in said cells, said electrode means in adjacent cells being energized relative to the strip with oppositely poled potentials, each of said cells 25 to the electrolyte inlet means and a substantially lesser current density at other portions of the electrode means, said current concentrating means including an electrode forming with said strip a gap which is narrowest at the part closest to said inlet means and which widens continuously to the other end of the electrode.

4. An apparatus as in claim 3, wherein said current concentrating means includes a second electrode forming with said strip a second gap which is narrowest at the inlet means and which widens to the other end of the second electrode, and which forms with said first electrode a continuously widening over-all gap from end to end of the electrodes.

5. An apparatus as in claim 4 wherein said pump means includes recirculating and regenerating means for recirculating the electrolyte for each cell and regenerating it.

References Cited UNITED STATES PATENTS 2,080,506 5/1937 lRinck et a1. 204 2,244,423 6/1941 Hall 204-l45 2,370,973 3/ 1945 Lang 204206 2,395,437 2/ 1946 Venable 204206 3,507,767 4/ 1970 Stricker 204208 JOHN H. MACK, Primary Examiner T. TUFARlELL-O, Assistant Examiner US. Cl. X.R. 

